1. Field of the Invention
The present invention relates to ophthalmic instruments, such as a non-contact type ophthalmotonometer, a keratometer, a refractometer, and a cornea endothelium camera, which need to align the main body with a subject eye to be tested.
2. Description of the Related Art
There is a conventional ophthalmic equipment provided with an index projection system and a light receiving system, as described in Japan Patent Publication No. H05-146410. The index projection system projects a collimated light beam onto the cornea of a subject eye in an oblique direction. The light receiving system has a light receiving element, such as a one-dimensional PSD (Position Sensing Detector), which receives the light beam reflected in an oblique direction symmetrical with the projection direction of the index projection light system with respect to the axis of the subject eye. Based on the position at which this light receiving element received the light beam, the ophthalmic equipment performs alignment in a Z direction.
However, in such an ophthalmic equipment, the collimated index light has a uniform light quantity distribution. For this reason, the quantity of incident light on the light receiving element is greater in the case where the main body of the instrument is considerably shifted away from an alignment reference position than in the case where the main body is located near the alignment reference position. This is because of the spherical aberration resulting from the convex shape of the cornea surface and because the spreading angle of reflected light becomes smaller in the case where the reflected light at the outer portion of the cornea surface is employed than in the case where the reflected light at the center portion is employed. In other words, as shown in FIG. 9(A), when cornea surface T is at an alignment reference position, a portion of an index light beam F, index light beam Fa, whose width is H2, is incident on the light receiving element. On the other hand, when the cornea surface T is shifted away from the alignment reference position (indicated by a broken line), as shown in FIG. 9(B), the width of the index light beam Fa incident on the light receiving element is H2, which is greater than the width H1. Thus, if the numerical aperture (NA) in the light receiving system is constant, the light receiving efficiency of the light receiving element will become greater in the case where the cornea surface T is considerably shifted away from the alignment reference position than in the case where the cornea surface is located near the alignment reference position.
Incidentally, in the case where a one-dimensional PSD is employed, a position at which an image is formed is computed by the following equation : EQU (I1-I2)L1/(I1+I2)=L2
where I1 is the current output from one end of the PSD, I2 is the current output from the other end of the PSD, L1 is the length of the PSD, and L2 is the position at which an image is formed on the PSD.
In the case where a position at which an image is formed is computed with the aforementioned equation, currents I1 and 12 are converted to voltages (0 to 5 V). The voltages are converted to digital numbers (0 to 255) by an A/D converter. From the digital numbers the image formed position is computed. From this computed image formed position, the position of the cornea relative to the alignment reference position is computed.
Since the A/D converter converts an analog value between the minimum voltage 0 V and the maximum voltage 5 V to a digital value of 0 to 255, currents I1 and I2 assume a value of 0 to 255, i.e., 256 kinds of values. For this reason, the minimum resolving power is 1/512, so the detection accuracy becomes high. In other words, to obtain the minimum resolving power 1/512, the A/D converter needs to convert the maximum voltage 5 V in correspondence to 255. This is because, for example, if 3 V is converted in correspondence to 255, then voltages equal to or greater than 3 V will be all 255 and therefore detection cannot be performed. Therefore, when the cornea position is near the alignment reference position, if the value of the current (I1+I2) is set to the maximum digital value 512, the quantity of incident light on the PSD will increase as the cornea shifts away from the alignment reference position, and therefore the value of the current (I1+I2) will increase. However, even if this current increased, the converted digital value of the current will remain 512 and therefore the position of the cornea (state of alignment) cannot be detected.
Therefore, there is a need to set the A/D converter so that when the denominator (I1+I2) becomes maximum, the value is the maximum digital value 512. More specifically, when the quantity ofincident light on the PSD becomes maximum, i.e., when a cornea shifts considerably away from the alignment reference position, the current (I1+I2) which is output from the terminal of the PSD must be set to the maximum digital value.
However, in the case where the maximum digital value 512 is set in the aforementioned manner, the quantity of incident light on the PSD will be reduced if a cornea is moved near the alignment reference position. Therefore, since the denominator (I1+I2) is reduced, detection accuracy decreases. In other words, there is a problem that at a position near the alignment reference position which requires high detection accuracy, the revolving power will be reduced and therefore the alignment detection accuracy will be reduced and, at a position considerably shifted away from the alignment reference position which does not require high detection accuracy, the revolving power will become high and therefore the alignment detection accuracy will become high.